1. Field of the Invention
This invention relates to a master information carrier carrying thereon an irregularity pattern representing the information to be transferred to a magnetic medium.
2. Description of the Related Art
With an increase in information quantity, there is a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc or a ZIP (Iomega) in the form of a flexible disc. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At the present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer). However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication Nos. 63 (1988)-183623, 2001-14667 and U.S. Pat. No. 6,347,016, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
The master information carrier employed in the magnetic transfer disclosed in Japanese Unexamined Patent Publication No. 2001-14667 and U.S. Pat. No. 6,347,016 comprises a patterned base sheet having on the surface thereof an irregularity pattern representing the information to be transferred and a magnetic layer formed at least on the surface of the protruding portions of the base sheet.
As a method of forming such a patterned base sheet, there has been known a method in which an irregularity pattern is formed on the surface of a non-magnetic flat plate by photolithography which has been employed in the field of semiconductor. However, it has been found that when the signal is transferred to a slave medium from a master information carrier comprising a non-magnetic base sheet, the quality of the transferred signal can greatly deteriorate due to asymmetry.
We, these inventors, have come to conceive that in the conventional master information carrier, where the base sheet is non-magnetic, since only the magnetic layer formed on the base sheet contributes to convergence of magnetic fluxes and convergence of magnetic fluxes concentrates on the magnetic layer, turbulence is generated in the magnetic field established on the slave medium side, which deteriorates symmetry (increases the degree of asymmetry). The magnetic layer on the base sheet is as thin as several tens to several hundreds nm, and the edges of the protruding portions are sharp. Due to such a small thickness of the magnetic layer and such a shape of the magnetic layer, the transfer magnetic layer is stabilized in the energy state at the edges of the protruding portions and concentration of magnetic fluxes on the edges occurs, which, we have determined, causes uniformity of the magnetic field in the area of transfer to deteriorate.
Further, we have determined that, in order to suppress turbulence of the magnetic field in the area of transfer of the slave medium, it is necessary to use a magnetic base sheet to rectify the transfer magnetic field passing through the slave medium.
However our further investigation has revealed that the transfer magnetic field in the area of transfer of the slave medium cannot be rectified by simply forming the base sheet itself of a magnetic material.